A lighting apparatus (hereinafter called an LED lighting apparatus) is known which is connected to an AC commercial power supply and used for lighting an LED (also called a light-emitting diode). Such LED lighting apparatus commonly operates by rectifying the power supplied from the AC commercial power supply. In particular, a pulsating or near-pulsating voltage may be applied across an LED array constructed by connecting a large number of LEDs in series without requiring the use of large capacitors.
If a pulsating voltage is directed applied to the LED array, the light emission period becomes short; to address this, it is known to provide a circuit for adjusting the number of series-connected LED stages by detecting the current flowing through the LED array (for example, refer to patent document 1).
FIG. 7 is a diagram showing an LED lighting apparatus illustrated in FIG. 26 in patent document 1. For convenience, FIG. 7 includes numbers, currents, etc. where necessary.
The LED lighting apparatus shown in FIG. 7 includes an AC commercial power supply 712, a bridge rectifier circuit 705 constructed from four diodes, a first LED group and a second LED group arranged in parallel, a third LED group connected in series to the first and second LED groups, resistors R1, R2, and R3, an n-type MOS transistor (FET) Q1, and an NPN transistor Q2.
The resistors R2 and R3 and the transistors Q1 and Q2 together constitute a bypass circuit 717. A current output terminal A of the bridge rectifier circuit 705 is connected to the parallel-connected first and second LED groups. The cathode side of the parallel-connected first and second LED groups is connected to the bypass circuit 717 as well as to the anode side of the third LED group. A current I3 passing through the bypass circuit 717 and a current I4 passing through the third LED group flow into the current sensing resistor R3 and the base of the transistor Q2 contained in the bypass circuit 717.
FIG. 8 is a diagram showing a voltage versus current relationship for the LED lighting apparatus of FIG. 7. FIG. 8(a) shows an example of a voltage waveform for one pulsating cycle that appears at the terminal A with respect to the terminal B of the bridge rectifier circuit 705, and FIG. 8(b) is an example of a current waveform for one pulsating cycle that flows in the bridge rectifier circuit 705. The current waveform shown in FIG. 8(b) is approximately equal to the sum of the currents I3 and I4.
The currents I3 and I4 are both equal to 0 A during a period t1 when the voltage at the terminal A is lower than the threshold voltage of the parallel-connected first and second LED groups. When the voltage at the terminal A subsequently rises and exceeds the threshold voltage of the parallel-connected first and second LED groups, the current increases rapidly for a short period t2. When the voltage at the terminal A further rises, there appears a period t3 during which the sum of the currents I3 and I4 is constant. In the first half of the period t3, only the current I3 flows through the bypass circuit 717, and in the second half of the period t3, the current I4 flows not only through the bypass circuit 717 but also through the third LED group. At this time, the currents I3 and I4 are regulated so that the base-emitter voltage of the transistor Q2 is maintained at 0.6 V.
Next, when the voltage at the terminal A rises, entering a period t4 which contains the peak of the voltage waveform, the transistor Q2 is saturated, and the bypass circuit 717 is cut off, so that the current I3 no longer flows. In the period t4, the overall current varies substantially linearly with the voltage of the terminal A, since the current I4 is only limited by the current-limiting resistor R3. The period during which the voltage of the terminal A falls is the reverse of the period during which the voltage rises.
The LED lighting apparatus of FIG. 7 has the advantage that, since the period t1 during which all the LEDs are turned off is short, not only does flicker decrease, but power factor and distortion factor both improve and harmonic noise also decreases.
In the prior art, it is also known to provide an LED lighting apparatus that includes a dimmer circuit between the AC commercial power supply and the bridge rectifier circuit (for example, refer to patent document 2). In the LED lighting apparatus disclosed in patent document 2, a pulsating voltage output from the bridge rectifier circuit is smoothed using a large-capacitance capacitor, and the thus smoothed voltage is used for lighting an LED.